Its not a jeep but maybe we can figure something out anyway. My other trick is a 89 Chevy c1500 with a 305. The exhaust manifold studs rotted off so I got a set of shorty headers to replace them. I put on the passenger side one first, then got busy so I drove it around on 1 header for a day or 2. It ran fine. I put on the other header saturday and that when the trouble started. It runs ok most of the time but when it shifts into od on the highway it starts missing and backfiring. If I drop it down into drive it runs fine. It also backfires around 70 at wot. The scanner says it has a code for running lean so i replaced the o2 sensor. The old one came out in pieces so something was wrong with it but it still runs the same with a new one. Anyone have any theories? it ran fine until I put on the drivers side header.

 

That’s cool. I just picked up a 91 c1500 over the winter. Only has the 4.3 though. Did you ever figure anything out with yours? I was going to suggest maybe part of the O2 harness hot melted by the exhaust.

 

It ended up being a combination of cheap plug wires and a bad fuel pump.